A custom API is the only way for a platform to extend its native E2EE sessions and features to other platforms. Making those APIs completely open would become a

A custom API is the only way for a platform to extend its native E2EE sessions and features to other platforms. Making those APIs completely open would become a major spam problem, which would likely end them up in the same situation as SMTP, where small servers are blocked-by-default by big providers. Interoperability by agreement between legitimate messaging services, using custom APIs is the only realistic and secure way to accomplish this.